Digital actuator with means to control piston acceleration and deceleration



R. M. cox ETAL 3,318,l96 DIGITAL ACTUATOH WITH MEANS TO CONTROL PISTON ACCELERATION AND DECELERATION 4 Sheets-Sheet 1 INV/:NTORS 25e/mw @NNE/wf. @e4/.5e www@ W] 5&4? 2 rfa/GIVE V5 f f Cf. .JFK f www WNY v am Sw., Ww\\ @055er M Cox 62 mi 6 May 9w 1967 Filed Nov. 27, 1964 4 Sheets-Sheet 2 ETAL NS TO CONTROL PISTO R. M. COX OR WITH MEA ACCELERATION AND DECELEHATION DIGITAL ACTUAT May 9, i?

Filed Nov. 27, 1964 May 9. 1967 R. M. cox ETAL 3,3%195 DIGT'I'AL ACTUATOR WITH MEANS TO CONTROL PISTON ACCELERATION AND DECELERATION Filed Nov. 27, 1964 4 Sheets-Sheet 20 FL 20 40052 f @Mam 3,318,196 L PIsToN May 9, 1967 A R. M. cox ETAL DIGITAL ACTUATOR WITH MEANS TO CONTRO' ACCELERATION AND DECELERATION Filed Nov. 27, 1964 4 Sheets-Sheet 4 United States Patent O 3,318,196 DIGITAL ACTUATOR WITH MEANS TO CONTROL PISTON ACCELERATION AND DECELERATION Robert M. Cox, Northridge, Clyde F. Czernek, Sepulveda,

and Kenneth F. Turner, Woodland Hills, Calif., as-

signors to CGC Associates, Inc., Van Nuys, Calif., a

corporation of California Filed Nov. 27, 1964, Ser. No. 414,371 17 Claims. (Cl. 91--168) This invention relates generally to digital actuators wherein a digital input signal is converted to a unique physical output displacement proportional to the real value of the digital input. More particularly, this invention relates to an improved digital actuator of the piston adder type.

In rece-nt years there has been an increasing trend towards the use of control systems and the like which utilize information in digital form. Consequently, there has been an attendant increase in the need for high precision digital actuators capable of producing an output displacement proportional to a digital input, eg., as in a process control valve system using digital input signals to regulate flow.

Such digital actuation can be accomplished by a wide variety of electrical, mechanical, pneumatic or hydraulic mechanisms, although hydraulic systems have heretofore been preferred in view of their generally enhanced durability and higher speed in comparison with the other types of digital actuation mechanisms. While such hydraulic digital actuators may embody either open loop systems or closed loop systems, the trend has been to the former since the latter are usually more expensive and pose problems with respect to the reliability of the feedback network. However, open lo-op digital actuation systems also pose some problems in view of their generally high power requirements and the excessively long settling time due to poor damping upon termination of the deceleration phase of actuator motion. In addition, some open lloop systems are subject to a certain amount of drift which results in reduced load positioning accuracy. Hence, those concerned with the development of digital actuators have long recognized the need for improved digital actuation devices capable of satisfying the high precision and high speed requirements of modern control systems. The present invention fulfills this need,

Accordingly, it is an object of the present invention to provide a new and improved digital `actuation system which overcomes the above and other disadvantages of the prior art.

Another object is to provide a new and improved digital actuator capable of extremely fast load positioning.

A further object of this invention is the provision of a new and improved digital actuator which is extremely durable.

Still lanother object is to provide a new and improved digital actuator capable of extremely accurate load positioning and precise repeatability.

Yet another object of the present invention is the provision of a new and improved digital actuator of extremely high efiiciency and hence, having relatively low power input requirements.

A still further object of this invention is the provision of a new and improved digital actuator of the piston adder type wherein substantially time optimal performance is provided for each individual piston adder and for all piston adders acting as -a group. Another object is to provide a new and improved piston adder digital actuation system wherein the energy input and energy output during acceleration and deceleration phases of motion are precisely controlled to elfect an increase in system efficiency, 'a decrease in stable load positioning time, and a reduction in drift and terminal motion settling time.

The above and other objects and advantages of this invention will become apparent from the following description, when taken in conjunction with the accompanying drawings of an illustrative embodiment thereof, and wherein:

FIGURE l is a combined electrical schematic and longitudinal sectional view through a presently preferred embodiment of a digital actuator in accordance with the present invention;

FIGURES 2a, 2b 'and 2c are enlarged, fragmentary sectional views of the adder piston and load portions of the digital actuator shown in FIGURE 1, and illustrate various stages of controlled adder piston motion during typical extension and retraction cycles of' operation;

FIGURE 3a is a schematic, free body force diagram for the acceleration phase of the extension cycle and the deceleration phase of the retraction cycle of the actuator system;

FIGURE 3b is a schematic, free body force diagram for the deceleration phase of the extension cycle and the acceleration phase of the retraction cycle of the actuator system;

FIGURES 4a, 4b and 4c are graphs indicative of the displacement, velocity and acceleration, respectively, versus time for the time optimal performance closely approached by all of the adder pistons and the load being positioned in the digital actuation system of the present invention; and

FIGURE 5 is a combined electrical s-chematic and block diagram of the digital control system for selectively extending and retracting individual `adder pistons in the actuator system.

Referring now to the drawings, and particularly to FIGURE 1 thereof, there is shown an electro-hydraulic digital actuator embodying the principles of the present invention. The digital actuation system includes a main block 10, typically of cast iron or the like, having a cylindrical main bore 12 therein extending a distance slightly less than the full length of the block. The bore 12 is open at one end 14 of the block 10 and closed at the opposite end 1-6 of the block. A plurality of cylindrical adder piston assemblies 101, 102, 104, 108, 116 and 132, representing the digital quantities 1, "2, 4, "8, "16, and 32, respectively, are slidably mounted within the bore 12 in coaxial alignment therewith. The piston assemblies lare typically fabricated of steel or the like, and both the bore 12 and the outside diameter of each piston assembly are precisely machined to close tolerances to permit relatively low friction sliding movement of each piston assembly along the bore 12 yet with minimal iiuid leakage across the various lands in the outer peripheral surface of each piston assembly.

All of the adder piston assemblies 101, 102, 104, 108, 116 and 132 are in end to end abutment within the bore 12 to form a series arrangement, and each piston assembly is capable of producing an incremental physical displacement of all of the remaining piston assemblies ahead of its leading end (to the right in FIGURE l) which is proportional to the real value of the digital input quantity represented by the particular piston assembly producing the displacement. In this connection, each of the adder piston assemblies 101, 102, 104, 108, 116 'and 132 is capable of producing an incremental output displacement proportional to each of the digital inputs 1, 2, 4, "8, 16 and 32, respectively. Hence, the adder piston assemblies within the bore 12 of the main block 10 together provide an incrementally extendable and retractable actuator column for selectively positioning an output shaft 18 which is aixed to and extends from a load piston 20 in abutment with the .leading end of the last adder piston assembly 132. The load piston 20 is slidably mounted within the bore 12 in coaxial alignment therewith, the bore 12 being of reduced diameter adjacent the end 14 of the block 10 to provide an annular shoulder 22 as a wall to contain the uid. In this regard, the opening 24 at the reduced diameter section of the bore 12 still provides adequate clearance for the output shaft 18, and fluid leakage through this opening is prevented by means of an appropriate seal 26. The load piston 20 produces a bias force acting on the adder piston assembly 132 which in turn applies this bias force to adder piston assemblies 116, 108, 104, 102, 101.

Considering the adder piston assembly 116 by Way of example, each piston assembly comprises an outer cylindrical sleeve 118 slidably received within the main bore 12, and an adder piston 120 mounted in close tting, sliding engagement vwithin a central longitudinal bore 122 of the sleeve 118. The trailing end 124 of the sleeve 118 is closed off, whereas the leading end 126 of the sleeve is provided with a clearance opening to permit the leading end 128 of the adder piston 120 to extend therethrough. In this connection, the leading end 128 of the adder piston 120 is of reduced diameter and provides an annular piston shoulder 130 adapted to lcooperate with an annular shoulder 134 at the leading end of the sleeve 118 to limit forward travel of the piston. Hence, the distance between the piston shoulder 130 and the sleeve shoulder 134 establishes the stroke length of the particular adder piston assembly, i.e., the distance that the leading end 128 of the adder piston can extend beyond the leading end 126 of its respective piston assembly sleeve 118.

All of the remaining adder piston assemblies 101, 102, 104, 108 and 132 are identical in their basic structural configurations with the aforedescribed adder piston ,assembly 116, the primary differences between the assemblies being in their respective adder piston stroke lengths so that each piston assembly is capable of producing a unique incremental output displacement proportional to the real value of the digital quantity represented by that particular piston assembly. In addition, the physical dimensions of the various porting arrangements may vary from one piston assembly to another in view of the differences in adder piston stroke length. However, these diiferences are only a matter of size, the operation of the porting arrangements of each piston assembly being exactly `the same as the operation of the corresponding porting arrangements for all of the remaining adder piston assemblies.

It will be apparent in FIGURE l that the leading end 128 of each adder piston 120 positions the trailing end 124 of the piston assembly sleeve directly ahead -of it. Similarly, the leading end of the adder piston within the latter sleeve likewise positions the sleeve of the piston assembly ahead of it, and so on for every adder piston assembly. Hence, the displacement of the load piston 20 and output shaft 18 along the main bore 12, relative to a zero position wherein all of the adder piston assemblies are fully collapsed, is always equal to the sum f the incremental displacements provided by every extended adder piston. In this regard, the maximum displacement which the digital actuation system is capable of providing is equal to the cumulative sum of all of the adder piston stroke lengths or, for the case illustrated, a physical displacement equal to the real value of a digital input 63. Of course, it will be appreciated that, while the digital actuator of the present invention has been illustrated for six piston assemblies capable of producing an output displacement proportional to the real value of any digital input between 0 and "63, the system can be readily modified to increase or decrease the number of adder piston assemblies in series and to thereby increase or decrease, respectively, the range of incremental output displacements which the system is capable of producing.

Operating pressures for the digital actuation system shown in FIGURE 1 are provided via three main hydraulic fluid lines 30, 40 and 50 in the block 10'. Supply pressure PS, typically 550 psig., is supplied to the fluid line 30 from a supply pressure accumulator (not shown). Return pressure PR, typically 45 p.s.i.g., is supplied to the Huid line y40 from a return pressure accumulator (not shown). A sump pressure PD of essentially 0 p.s.i.g. is supplied to the uid line 50.

The leading end of each adder piston assembly is vented to the sump line 50 by means of a plurality of uid lines 60 extending from the sump line and communicating with the main bore 12 of the block 10. In this connection, since the leading end of each of the adder piston assemblies may assume a wide range of different physical positions along the main bore 12, an annular passageway, such as the annular passageway 136 at the leading end of the adder piston assembly 116, is machined into the outer peripheral surface of the leading end of the sleeve 118 and the trailing end of the piston sleeve ahead of it. The width of the annular passageway 136 at the leading end of each piston assembly is always vented via the annular passageway and one of the lines 60 to the sump line 50. Of course, it will be apparent that the annular passageways such as 136 may be machined into the bore 12 of the block 10 rather than into the outer peripheral surface of each piston assembly sleeve, depending upon the economics of manufacture involved.

The supply pressure and return pressure lines 30 and 40, respectively, provide fluid pressure inputs to a bank of parallel pressure control systems 201, 202, 204, 208, 216 and 232 corresponding to the adder piston assemblies 101, 102, 104, 108, 116 and 132, respectively. While all of the adder piston assemblies are connected in series and are, therefore, mutually dependent upon each other for their exact positions along the main bore 12, all of the pressure control systems are connected in parallel and are, hence, independent of each other.

Each adder piston assembly is provided with its own pressure control system, e.g., the piston assembly 116 working in conjunction with its respective pressure control system 216, the latter system communicating with the main bore 12 adjacent the piston assembly 116 via a pair of uid lines 140 and 142.

Each of the pressure control systems 201, 202, 204, 208, 216 and 232 is physically identical to every other pressure control system, and each pressure control system is capable of being conditioned -to one of two possible control states by its own solenoid 301, 302, 304, 308, 316 or 332, respectively. Each of these solenoids, e.g., the solenoid 316, has an energizing winding 340 and a solenoid plunger 342, the solenoid plunger assuming one of two possible positions (the solid and phantom positions indicated for the solenoid 316 in FIGURE l) depending upon whether or not an electrical voltage is f applied to the solenoid as an electrical ON signal or -as an electrical OFF signal corresponding to the binary l or bin-ary 0 state, respectively, of the particular digital quantity represented by the solenoid.

When the input to the solenoid winding is off, the pressure control system associated with that particular solenoid is conditioned in such a manner that its associated adder piston assembly is forced into the fully retracted state. When the digital input to the solenoid winding is on, the solenoid plunger conditions its associated pressure control system so that the associated adder piston assembly is forced to assume its fully extended state. In accordance with the present invention, extension and retraction within each adder piston assembly is accomplished under controlled energy conditions such that very little energy is lost by the actuation system during each extension and retraction cycle.

It is to be understood that, while solenoids are illustrated as a presently preferred means for selectively conditioning the pressure control system for each adder piston assembly, this is by Way of example only, and other selective conditioning means, e.g., of the pneumatic or mechanical types, may be utilized in place of solenoids without departing from the spirit and scope of the present invention. In addition, while the specic digital actuation system herein described makes use of a hydraulic fluid such as oil or other appropriate liquid, the concepts of the invention are also applicable to systems using compressible tluid media.

The portion of the actuation system for a digital r16 input, including the solenoid 316, pressure control system 216 (shown within dotted lines in FIGURE .1) and adder piston assembly 116 is typical, and all of the ensuing description with respect to the operation of this portion of the actuation system is equally applicable to the solenoids and associated pressure control systems and adder piston assemblies for all of the other digital inputs. p

Referring now more particularly to FIGURES l and Zar-2c, the operation of the actuation system for both extension and retraction proportional to a digital 16 value is next described. In this connection, it will be noted in FIGURES 2li-2c that the adder piston of the piston assembly S is shown in its fully extended position. However, the operational analysis for the adder piston assembly 116 is the same regardless of the particular position of the sleeve 118 along the main bore 12 dictated by 4the extended or retracted states of all of the piston assemblies behind it (to the left in FIGURES 1 and 12a-2c).

The outer peripheral surface of the piston assembly sleeve 118 is machined to provide a pair of annular passageways 144 and 146 adjacent the uid lines 140 and 142, respectively. The annular passageways 144 and 146 are sealed off from each other by an annular rib or land 14S between them. The outer lands 150 and 152 dening the annular passageways 144 and 146, respectively, serve to seal off the passageways so that theyare not vented to the sump line 50 via one of the iluid lines 60. As in the case of the annular passageway 136 previously described, the width of each of the passageways 144 and 146 is tailored for each -individual adder piston assembly so that the passageway 144 is in constant communication with the fluid line 140 and the passageway 146 is in constant communication with the fluid line 142, irrespective of the position of the particular adder piston assembly along the main bore 12.

Each of the annular passageways 144 and 146 in the outer peripheral surface of the piston assembly sleeve 118 communicates with the interior bore 122 of the sleeve by means of a port 154, 156, respectively. These ports 154, 156 are separated from each other along the surface of the bore 122 by a land 15S. The width of the land 158 is slightly less than the width of a port 160 in the adder piston 120, the port 160 being in constant fluid cornmunication with a driving cavity 162 of the adder piston and one or the other of the ports 154, 156 extending through the sleeve 118.

The fluid lines 140 and 142 are in communication with the bore on opposite sides of the sealing piston 240 of a double-action ball valve 242 within the pressure control system 216 for the adder piston assembly 116. The double-action valve 242 is controlled by the solenoid plunger 342 of the solenoid 316. Hence, the doubleaction valve 242 has two positions, indicated by the solid and phantom line positions of the Valve in FIGURE l, corresponding to the two positions of the solenoid plunger 342 indicated by the solid and phantom positions of the plunger, respectively.

One end 244 of the double-action valve 242 communicates with the return pressure line 40 by means of a fluid line 246, and the two positions of the ball valve at this end therefore selectively communicate or cut off the fluid line 142 from the return pressure line.

Similarly, the opposite end 248 of the doubleaction valve 242 is in iluid communication with the supply pressure line 30, and the two positions of the ball valve at this latter end selectively communicate or cut off the uid line 140 from the pressure supply line.

A fluid line 250 bypasses the line 142 around the end 244 of the valve 242 to the return pressure line 40. A check valve 252 is included in the line 250 and is spring biased so that it will lift olf its seat only when the pressure in the line 142 falls below the return pressure PR. p

Similarly, a line 254 is in fluid communication with the line 140 and bypasses the end 248 of the valve 242 to the supply pressure line 30. A check valve 256 is included in the line 254 and is spring biased so that the ball will lift oft its seat only when the pressure in the line 140 exceeds the supply pressure PS.

An inter-port bleed orifice 258 communicates the uid lines 140 and 142 so that, under static conditions, the fluid pressures n these lines will equalize,

Prior to a digital 16 ON input to the solenoid 316, the solenoid plunger 342 and double-action valve 242 are in their solid positions shown in FIGURE l. The adder piston is fully retracted within the sleeve 118 of the adder piston assembly 116, and the piston drive cavity 162 is at the return pressure level PR to bias the leading end 128 of the adder piston against the trailing end of the piston assembly sleeve ahead of it (FIGURE 2a).

As soon as an ON input signal is fed to the winding 340 of the solenoid 316, the solenoid plunger 342 and double-action valve 242 move to their phantom positions shown in FIGURE 1. 5

When the double-action valve 242 moves to its phantom positions, the line 142 is cut off from the return pressure line 40. However, the pressure in line 142 will never fall below the return pressure PR because: of the return pressure check valve 252. At the same time, uid line is connected to the supply pressure line 30. Hence, the pressure in the adder piston drive cavity 162 is raised from return pressure PR to full supply pressure PS via the line 140, passageway 144, sleeve port 154 and adder piston port 160. Since the output displacement end of the adder piston assembly 116 is always vented directly to the sump line 511 at zero p.s.i.g., the diierential pressure across the adder piston 120 is thus equal to the full supply pressure PS. This produces an extension driving force on the adder piston 120 equal in magnitude to the product of the cross-sectional area of the adder piston and the supply pressure PS.

The forces opposing extension of the adder piston 120 are those of friction (essentially negligible) and the force exerted by the load piston 2u back on the piston assembly actuator column. In this connection, and referring to FIGURE 1, the main bore 12 on the side of the load piston 20 opposite that in contact with the piston assembly actuator column is constantly vented by a line 31 to the supply pressure line 30. However, for energy control considerations which will hereinafter be explained, the effective load piston area upon which the supply pressure PS acts is just slightly more than one-half of the effective cross-sectional area of the adder piston 120. Hence, the net driving force acting to accelerate the adder piston 121D, all of the adder piston assemblies ahead of the assembly 116 in the actuator column, the load piston 21.! and the load (not shown) connected to the output shaft 18 is eoual to slightly less than one-half of the product of the adder piston area and the supply pressure PS. The acceleration produced by this net driving force continues` for the first half stroke of the extension cycle, until the adder piston 120 reaches its mid-stroke position shown in FIGURE 2b.

As the adder piston 120 passes through its mid-stroke extension position, the port 154 which had been supplying full supply pressure to the adder piston port and drive cavity 162 is cut 01T. This immediately removes the acceleration force which had been acting upon the adder piston for the first half cycle. However, as the trailing edge of the port 160 cuts off the port 154 by passing the land 158, the leading edge of the port 160 comes into communication with the port 156 to admit return pressure PR to the piston drive cavity 162.

Since PR is So much less than Ps, the net force acting upon the adder piston 120 and the load is essentially just slightly less than the product of the effective load piston area and the supply pressure Ps. Hence, as soon as the port 154 -is cut olf, the load ,and the adder piston immediately begin to decelerate. In this connection, the load piston 20 is sized in such a Iway as to provide a net deceleration force which is exactly equal in magnitude to the net acceleration force which acted during the first half of the extension cycle. Therefore, when the acceleration force is removed, the net force acting upon the load is exactly equal and opposite to that which had been previously applied, with the consequent result that the deceleration rate is equal to the acceleration rate to provide time optimal performance. In this connection, since the load piston 20 is always directly connected by the line 60 to the supply pressure line 30, the energy initially imparted by the adder piston 120 during the first half of the extension cycle is returned to the supply pressure accumulator by the displacement of the load piston during the second half of the extension cycle. In effect, during the second half of the extension cycle when the system is in the decleration phase, the load piston Ztl becomes a pump with the load (not shown) supplying the driving force.

Referring now to FIGURE 2c, the velocity of the adder piston 120 is reduced to zero just before the piston achieves its maximum extension stroke displacement. When the adder piston velocity reaches Zero, there is a tendency for the piston to reverse its direction of movement and accelerate in the opposite direction due to the load piston force being applied. However, as soon as the load piston 20 attempts to drive the adder piston 12) backwards, the return pressure check valve 252 (FIG- URE l) closes off the line 25) to dead end the adder piston drive cavity 162. Hence, the pressure `in the drive cavity 162 rapidly rises to the full supply pressure PS through the bleed orifice 258 which communicates the supply pressure line 30 with line 142, passageway 146 and sleeve port 156. This rise in pressure causes the adder piston 120 to complete the minute remaining increment of the extension stroke and, at the same time, provides a holding `force on the adder piston opposing the force produced by the load piston 20. In this manner, the actuation system is now locked up in the extended mode for the digital 16 input signal.

The reason for supplying return pressure PR to the adder piston drive cavity 162 during the decleration phase of the extension cycle is to keep all of the adder piston assemblies in contact with each other and ywith the load piston during the deceleration phase. Otherwise, viscous forces and the like may introduce sutiicient dra-g to cause the piston :assemblies to lose contact with each other and with the load piston.

The retraction cycle for the `adder piston assembly 116 can `be analyzed in the same manner. The first event which occurs is the de-energization of the solenoid 316 to move the double-action valve 242 to its solid position shown .in FIGURE 1. This immediately causes the adder piston cavity 162 to be vented to the return pressure line 40. The adder piston 120 then accelerates from its position .shown in FIGURE 2c to its mid-stroke retraction position shown in FIGURE 2b. The accelerat- .ing force for this first half of the retraction cycle is exactly equal to the decelerating force applied to the adder piston 120 during the second half of the extension cycle.

When the adder piston 120 reaches the mid-stroke position, the port 56 is cut off from the port 160, and the port 154 is brought into communication with the adder piston cavity 162 via the port 160. However, yline 140 is now cut olf `from the supply pressure line 30 by the retraction position of the double-action valve 242. Hence, the driving cavity 162 is capped off, and the pressure immediately rises until it `reaches a level equal tothe supply pressure PS. At this point, the spring restralned supply pressure ball of the check valve 256 lifts off its seat and allows -hydraulic fluid to be returned to the supply pressure accumulator. With the pressure in the adder piston cavity 162 now equal to full supply pressure PS, a deceleration force is exerted upon the adder piston and the load which is equal in magnitude to the acceleration force which was applied during the first half of the extension cycle.

The adder piston 120 reaches zero velocity just slightly before completion of the retraction stroke (FIGURE 2a). At this point, the pressure in the adder piston cavity 162 drops off to the return pressure level through the bleed orifice 258. The load piston 2t? then drives the adder piston 120 the remaining minute incremental distance necessary to complete the retraction stroke, yat which time the leading end 126 of the sleeve 118 prevents any further forces from being imparted by the load piston to the leading end 128 of the adder piston. The return pressure in the adder piston drive cavity 162 then serves to bias the adder piston 12@ so that its leading end is in abutment with the trailing end of the piston assembly sleeve ahead of it in the actuator column.

As previously indicated, the effective piston area of the load piston 2t) upon which the supply pressure PS acts iS sized such that, iwith supply pressure applied to the adder piston during half of any extension or retraction cycle and return pressure applied to the adder piston during the other half of the same cycle, the net acceleration and deceleration forces will always be of equal magnitude so that the conservation of energy `and time optimal performance characteristics ofthe actuation system can be achieved. The specific ratio of effective load piston area to effective adder piston area required in order to obtain this equality between net acceleration and net deceleration forces will be apparent from the following analysis in connection with FIGURES 3a and 3b.

FIGURE 3a is a free body diagram of the adder piston and load piston 20 treated as a single unit upon Which the system forces act. This configuration is valid since the actuator column and load piston are always in constant contact with each other during the acceleration and deceleration phases. FIGURE 3a covers the acceleration phase of the extension cycle or, alternatively, the deceleration phase of the retraction cycle. If the effective piston area of the adder piston 120 is A and the effective piston area of the load piston 20 is L, then the net acceleration force upon the free body in FIGURE 3a is (A-L) Psi.

FIGURE 3b illustrates the forces acting upon the same free body as that inV FIGURE 3a but during the deceleration phase of the extension cycle or, alternatively, the acceleration phase of the retraction cycle. The net force acting upon the free body in FIGURE 3b is LPs--APr I In order to achieve time optimal performance and the desired conservation of energy for both the extension and retraction cycles, the net forces of FIGURES 3a and 3b must be exactly equal in magnitude. Hence, the following relationships must hold true: f

@FILA-Aal 2) where F3a=net force in FIGURE 3a; and F3b=net force in FIGURE 3b F3a=Fab (3) Therefore,

APs--LP .,=LPs-APr (4) and It was previously pointed out that the effective piston area L of the load piston 20 is slightly greater than one-half of the effective adder piston area A. The reason for this will be apparent from the following analysis.

Solving Equation 6 for Pr yields:

2L 2L P,P,-P, P,(-1 (7) Since Pr must be greater than zero in order to properly bias the adder pistons into contact with the load piston during deceleration, therefore 2L/A 1 (9) and Hence, the effective piston area of the load piston 20 will preferably always be greater than one-half of the effective adder piston area. In the foregoing analysis, all pressures 'are measured relative to the sump pressure PD.

The performance of the adder piston assemblies and the load closely approaches the time optimal displacement, velocity and acceleration curves of FIGURES 4a, 4b and 4c, respectively. Settling problems at the end of each extension stroke are virtually nil since the kinetic energy of the moving system is essentially reduced to zero at the same time as the extension stroke is completed. Hence, there is little or no energy to be given up by way of impact at the end of the extension stroke.

The` basic premise upon which the digital actuation system of the present invention operates is that the load must move in a time optimal manner in order to conserve the system energy and hence control the motion. A direct corollary to this hypothesis is that, if the motion of the adder pistons is to take place in both directions, all movement in one `direction (either retraction or extension) must be completed before movement in the other direction can take place. In order to exercise this type of control, a` logic network is incorporated into the system to sense whether any change in load position is desired, sense whether or not the change is in the retraction or extension direction of the actuator column, and time delay either the expanding or retracting groups of adder piston assemblies.

The required logic network for enabling all motion in one direction to be completed before motion in the opposite direction is initiated is shown in FIGURE 5. Each of the solenoids 3611, 362, 304, 308, 316 and 332` is conditioned to its extension or retraction position depending upon the positive or negative state, respectively, of its associated control flip-flop 401, 402, 404, 498, 416 or 432, respectively. These control flip-flops are conditioned to their particular states by the positive or negative signals received via signal lines 5011, 5G12, 504, 50S, 516 or 532, respectively, which represent the binary output from a digital computer section 550.

Two opposite polarity unidirectional input paths for each of the signal lines to its associated flip-Hop are provided by means of a pair of oppositely oriented diodes in each pair of input paths to each Hip-Hop. These'are the diodes 601-632 and 791-732., respectively. However, one of the pair of input paths to each control flip-flop (the negative input path in the embodiment illustrated in FIGURE 5) is provided with a series of normally disabled gates 801-8132, respectively. Each of these gates is connected to receive as an enabling input the output S50 from a delay line 852 or the like.

Whenever a change in actuator position is desired, the digital computer produces a seek signal which is simultaneously directed over lines 854 and 856 to the section 550 and delay line 852, respectively. The seek signal enables the gates in the section 550 so that the new actuator position address input can alter the states of the binaries in the section 550' and produce corresponding positive and negative signals over lines 501, 502, 504,v

S08, $16 and 532.

The positive signals are passed directly to the control flip-Hops 401, 402, 404, 408, 416 and 432.* by the diodes 701, 702, 704, S, 716 and 732, respectively, in the positive input paths to each control flip-flop. However, the normally disabled gates 81111-8312 in the negative input path to each control fiip-op prevent the passage of negative inputs until the seek signal appears at the output S50 of the delay line 352. 8011-832 is enabled to pass any negative input available over the associated signal lines 50\1-53i2, respectively. Hence, all movements of the actuation system in one direction will be completed before movements in the opposite direction are initiated, as long as the time delay of the delay line 852 is sufiiciently long in duration to permit all of the actuation movements in one direction to be completed Ibefore the seek signal can enable the gates 801-832.

The digital actuation system of the present invention exercises precise control over the amount of energy which is both imparted and extracted from the driven load. By making the imparted and extracted energy equal in both the extension cycles and the retraction cycles of the actuation systern, the eiciency of the system is extremely high due to the energy conservation. Moreover, the closely matched acceleration and deceleration phases of each cycle results in rapid settling of the system at the end of extension and retraction strokes without exciting the natural frequency of the mechanical structure which is either being driven or forms an attaching point for the actuator. At the same time, the actuation system is very durable and is capable of extremely fast positioning and a high degree of positioning accuracy.

It will be apparent from the foregoing that, while a particular form of our invention has been illustrated and described, various modications can be made without departing from the spirit and scope of our invention. Accordingly, we do not intend that our invention be limited, except as by the appended claims.

We claim:

1. In a digital actuation system, the combination cornprising:

an adder piston;

a load piston coaxial with said adder piston and in faceto-face contact therewith;

and means for accelerating and decelerating said adder piston and said load piston in either direction along their common axis including means for producing 011 said load and adder pistons forces which are exactly equal in magnitude but opposite in direction during the accelerating and decelerating phases, whereby time optimal performance of said adder piston and said load piston during said accelerating and decelerating phases is obtained.

2. In a digital actuation system, the combination comprising:

at least one adder piston;

a yload piston coaxial with said adder piston and in face-to-face contact therewith;

and means for applying a rst net force to said adder piston and said load piston to accelerate said adder piston and said load piston over a first predetermined displacement distance along their common axis and means for applying a second net force equal in mag- At that time, each of the gates nitude and opposite in direction to said first net force to said adder piston and said load piston to decelerate said adder piston and said load Ipiston over a second displacement distance equal to said first distance, whereby time optimal movement of said adder piston and said load piston is obtained.

3. In a digital actuation system, the combination comprising:

an adder piston;

a load piston coaxial with said adder piston and in faceto-face contact therewith;

means for accelerating and decelerating said adder piston and said load piston in either direction along their common axis including means for producing on said load and adder pistons forces which are exactly equal in magnitude but opposite in direction during the accelerating and decelerating phases, whereby time optimal Vperformance of said adder piston and said load piston during said accelerating and decelerating phases is obtained;

and means for conserving the energy used in each acceleration phase of said adder piston and said load piston for use in each deceleration phase of said adder piston and said load piston and vice versa.

4. In a digital actuation system, the combination cornprising:

an adder piston;

a load piston coaxial with said adder piston and in faceto-face contact therewith, said load piston having an effective piston area which is less than the effective piston area of said adder -piston but greater than onehalf the effective piston area of said adder piston;

means for applying a first net force to said adder piston and said load piston to accelerate said adder piston and said load piston over a rst predetermined displacement distance along their common axis and means for applying a second net force equal in magnitude and opposite in direction to said first net force to said adder piston and said load piston to decelerate said adder piston and said load piston over a second displacement distance equal t said first distance;

and means for conserving substantially all of the energy used in each acceleration phase of said adder piston and said load piston for re-use in each deceleration phase of said adder piston and said load piston and vice versa.

5. In a digital actuation system, the combination comprising:

an adder piston;

a load piston coaxial with said adder piston and in faceto-face contact therewith, said load piston having an effective piston area which is less than the effective 'piston area of said adder piston;

a supply pressure source;

a return pressure source, said return pressure being less than said supply pressure;

means for applying supply pressure to said load piston at all times to resist movement of said load piston by said adder piston;

means for applying supply pressure to said adder piston to accelerate said adder piston and said load piston in a direction along their common axis opposite to the forces of said supply pressure acting upon said load piston;

and means for interrupting the application of supply pressure to said adder piston and for applying return pressure to said adder piston whereby said adder piston and said load piston are decelerated, the relative piston areas of said load piston and said adder piston being such that the net deceleration force acting upon the dynamic system is equal in magnitude to the net acceleration force acting upon said system, whereby substantially all of the energy supplied by said supply pressure source during acceleration of said load piston and said adder piston is returned to l2 said supply pressure source during deceleration of said adder piston and said load piston. 6. In a digital actuation system, the combination comprising:

an adder piston;

a load piston coaxial with said adder piston and in face-to-face contact therewith, said load piston having an effective piston area which is less than the effective piston area of said adder piston;

a supply pressure source;

a return pressure source, said return pressure being less than said supply pressure;

means for applying supply pressure to said load piston at all times to resist movement of said load piston by said adder piston;

means for applying return pressure to said adder piston to allow said adder piston and said load piston to accelerate along their common axis in the direction of the force provided by said supply pressure acting upon said load piston;

and means for interrupting the application of return pressure to said adder piston and for applying supply pressure to said adder piston whereby said adder piston and said load piston are decelerated, the relative piston areas of said load piston and said adder piston being such that the net deceleration force acting upon the dynamic system is equal in magnitude to the net acceleration force acting upon said system, whereby substantially all of the energy supplied by said supply pressure source during acceleration of said load piston and said adder piston is returned to said supply pressure source during deceleration of said load piston and said adder piston.

7. In a digital actuation system, the combination cornprisin g an adder piston;

a load piston coaxial with said adder piston and in faceto-face contact therewith;

a supply pressure source;

a return pressure source, said return pressure being less than said supply pressure;

means for applying supply pressure to said load piston at all times to resist movement of said load piston by said adder piston;

and means for selectively applying supply pressure or return pressure to said adder piston to control the acceleration and deceleration of said adder piston and said load piston such that substantially all of the energy supplied by said supply pressure source during acceleration of said adder piston and said load piston is returned to said supply pressure source during deceleration of said adder piston and said load piston, the relative piston areas of said load piston and said adder piston being such that the net deceleration force acting upon the dynamic system is equal in magnitude to the net acceleration force acting upon said system,

`8. A digital actuation system, comprising:

a supply pressure source;

a return pressure source, said return pressure being less than said supply pressure;

an adder piston; Y

a load piston coaxial with said adder piston and in face-to-face contact therewith, the ratio of the effective load piston area to the effective adder piston area being Ps-i-PR 213s where P5 is the supply pressure and PR is the return pressure;

means for applying supply pressure to said load piston at all times to resist movement of said load piston by said adder piston;

9. A digital actuation system, comprising:

t at least oneadder piston assembly having a sleeve and anl adder piston slidably mounted within said sleeve Iand movable between extended and retracted positions, said adder piston having a driving face;

a load piston coaxialV with said adder piston assembly and in Contact with the adder piston thereof, the face of said load piston opposite that in contact with said adder piston having an effective piston area which is less than the effective piston area of said adder piston driving face but greater than one-half the effective piston area of said adder piston driving face;

a first source of supply pressure;

a second source of return pressure lower than said supply pressure;

means for applying supply pressure from said first source to said face of said load piston at all times;

means for applying supply pressure from said first source to said driving face of said adder piston for the li-rst half of each extension stroke and the second half of each retraction stroke;

and means for applying return pressure from said second source to said driving face of said adder piston for the second half of each extension stroke and the first half of each retraction stroke.

10. A digital actuation system, comprising:

at least one adder piston assembly having a sleeve and an adder piston slidably mounted within said sleeve and movable between extended and retracted posip tions, said adder piston having a driving face;

a load piston coaxial with said adder piston assembly and in contact with the adder piston thereof, the face of said load piston opposite that in contact with said adder piston having an effective piston area which is lless than the effective piston area of said adder piston driving facebut greater than one-half the effective piston area of said adder piston driving face;

i i; a first source of supply pressure; i

a second source of return pressure lower than said supply pressure;

`means for applying supply pressure from said first source to said face of said load piston at all times;

means for applying supply pressure from said first source to said driving face of said Iadder piston for the firsthalf of each extension stroke and the second half of each retraction stroke;

means for applying return pressure from said second source to said driving face of said adder piston for the second half of each extensionl stroke and the first half of each retraction stroke; p

means for increasing the pressure applied to said driving face of said adder piston 'from return pressure to supply pressure at the termination of each extension stroke;

and means for decreasing the pressure applied to said driving face of said adder piston from supply pressure to return pressure at the termination of each `retraction stroke.

11. A digital actuation system, comprising:

a block havingia main cylindrical bore therein;

a source of supply pressure PS;

a source of return pressure PR which is lower than said supply pressure;

`at least one cylindricaladder piston assembly within said main bore, said piston assembly having a sleeve and an adder piston slidably mounted within said sleeve and movable between extended and retracted positions, said adder piston having a driving face;

a cylindrical load piston coaxial with said adder piston assembly and in contact with the adder piston thereof, the face of said load piston opposite that in contact with said adder piston having an effective piston area which is less than the effective piston area of said adder piston driving face, the ratio of said load piston area to said adder piston area being Ps'l-PR 2PS means for applying supply pressure to said face of said load piston at all times;

means for applying supply pressure to said driving face of said adder piston for the first half of each extension stroke and the second half of each retraction stroke;

means for applying return pressure to said driving face of said adder piston for the second half of each extension stroke and the first half of each retraction stroke;

means for increasing the pressure applied to said driving face of said adder piston from return pressure to supply pressure at the termination of each extension stroke;

and means for decreasing the pressure applied to said driving face of said adder piston from supply pressure to return pressure at the termination of each retraction stroke.

l2. A digital actuator, comprising:

a housing having `a main cylindrical bore therein;

a source of supply pressure PS;

a source of return pressure PR less than said supply pressure;

a plurality of coaxial adder piston assemblies in endto-end abutment within said bore, each piston assembly including a cylindrical sleeve slidably mounted within said bore and an adder piston slidably mounted within said sleeve and movable between extended and retracted positions, the leading end of each sleeve having an opening therein and the trailing end of each sleeve being closed, each piston having a driving face at the trailing end of said piston and the leading end of each piston being of reduced diameter to clear said opening in its associated sleeve, the length of the reduced diameter portion of each piston being suc-h that in the extended position the leading end of said piston extends beyond the leading end of its associated sleeve a distance which is proportional to the real value of a digital quantity represented by the particular adder piston assembly, the leading end of each piston being flush with the leading end of its associated sleeve in the retracted position of that piston;

a load piston within said main bore and coaxial with said piston assemblies, said load piston being in abutment with the leading end of the nearest adder piston, the face of said load piston opposite that in contact with said nearest adder piston having an effective piston area relative to the effective piston area of the driving face of each adder piston in the ratio of Psi-PR 2PS i means for applying supply pressure to said main bore to 'bias said Vload piston against said adderpiston assemblies at all times;

means for applying supply pressure to the driving face of an adder piston for the first half of each extenl sion stroke and the second half of each retraction stroke;

means for applying return pressure to said driving face of said adder piston for the second half of each extension stroke and the first half of each retraction stroke;

and means for preventing initiation of any strokes in one direction before all strokes in the opposite direction have been completed.

13. A digital actuator, comprising:

a housing 'having a main cylindrical bore therein;

a source of supply pressure PS;

a source of return pressure PR less than said supply pressure;

a plurality of coaxial adder piston assemblies in endto-end abutment within said bore, each piston assembly including a cylindrical sleeve slidably mounted Within said bore and an adder piston slidably mounted within said sleeve and movable between extended and retracted positions, the leading end of each sleeve having an opening therein and the trailing end of each sleeve being closed, each piston having a driving face at the trailing end of said piston and the leading end of each piston being of reduced diameter to clear said opening in its associated sleeve, the length of the reduced diameter portion of each piston being such that in the extended position the leading end of said piston extends beyond the leading end of its associated sleeve a distance which is proportional to the real value of a digital quantity represented by the particular adder piston assembly, the leading end of each piston being flush with the leading end of its associated sleeve in the retracted position of that piston;

a load piston wit-hin said main `bore and coaxial with said piston assemblies., said load piston being in abutment with the leading end of the nearest adder piston, the face of said load piston opposite that in contact with said nearest adder piston having an effective piston area relative to the effective piston area of the driving face of each adder piston in the ratio of means for applying supply pressure to said main bore to bias said load piston against said adder piston assemblies at all times;

means for applying supply pressure to the driving face of an adder piston for the lfirst half of each extension stroke and the second half of each retraction stroke;

means for applying return pressure to said driving face of said adder piston for the second half of each extension stroke and the first half of each retraction stroke;

means for increasing the pressure applied to said driving face of said adder piston from return pressure to supply pressure at the termination of eac-h extension stroke;

means for decreasing the pressure applied to said driving face of said adder piston from supply pressure to return pressure at the termination of each retraction stroke;

and means for preventing initiation of any strokes in one direction before all strokes in the opposite direction have been completed.

14. A digital actuator, comprising:

a housing having a main cylindrical bore therein;

a source of supply pressure PS;

a source of return pressure PR pressure;

a sump at substantially zero p.s.i.g.;

a plurality of coaxial adder piston assemblies in endtO-cnd abnimmt Within said bore, each piston asless than said supply sembly including a cylindrical sleeve slidably mounted within said bore and an adder piston slidably mounted within said sleeve and movable between extended and retracted positions, the leading end of each sleeve having an opening therein and the trailing end of each sleeve being closed, each piston having a driving face at the trailing end of said piston and the leading end of each piston being of reduced diameter to clear said opening in its associated sleeve, the length of the reduced diameter portion of each piston being such that in the extended position the leading end of each adder piston extends beyond the leading end of its associated sleeve a distance Which is proportional to the real value of a digital quantity represented by the particular adder piston assembly, the leading end of each piston being iiush with the leading end of its associated sleeve in the retracted position of that adder piston;

a load piston within said main bore and coaxial with said piston assemblies, said load piston being in abutment with the leading end of the nearest adder piston, the face of said load piston opposite that in contact with said nearest adder piston having an effective piston area relative to the effective piston area of the driving face of each adder piston in the ratio of Ps-i-PR 213B means for communicating the leading end of every adder piston assembly with said sump;

means for applying supply pressure to said main bore to bias said load piston against said adder piston assemblies at all times;

first and second port means in each piston assembly sleeve;

fluid bleed means communicating said second port means with said third port means;

first check valve means for venting said first port means to said source of supply, pressure whenever the pressure at said first port means exceeds the supply pressure;

second check valve means for venting said second port means to said source of return pressure whenever the pressure at said second port means falls below said return pressure;

third port means in each adder piston in fluid communication with the driving face of the particular adder piston, said third port means being in fluid communication with said tirst port means during the first half of each extension cycle and the second half of each retraction cycle, said third port means being in fluid communication with said second port means during the second half of each extension cycle and the first half of each retraction cycle;

a plurality of actuator devices, one for each of said adder piston assemblies;

and a two-position valve for each adder piston assembly, each valve being controlled by the actuator device for that particular piston assembly, said valve in one position applying supply pressure directly to said rst port means, said valve in the other position supplying return pressure directly to said second port means, whereby to selectively induce acceleration and deceleration phases of said adder piston and said load piston for each extension and retraction cycle.

15. A digital actuator as set forth in claim 14, wherein the actuator device for each adder piston assembly is a solenoid connected to the two-position valve for that particular adder piston assembly.

16. A digital actuator as set forth in claim 14, including means for preventing initiation of any adder piston strokes in one direction before all of the adder piston strokes in the opposite direction have been completed.

17. In an electro-hydraulic digital actuation system, the

combination comprising:

a housing having a main cylindrical bore therein;

a source of hydraulic supply pressure PS;

a source of hydraulic return pressure PR less than said supply pressure;

a sump at substantially zero p.s.i.g.;

a plurality of coaxial adder piston assemblies in endto-end abutment within said bore, each piston assembly including a cylindrical sleeve slidably mounted within said bore and an adder piston slidably mounted within said sleeve and movable between extended and retracted positions, the leading end of each sleeve having an opening therein and the trailing end of each sleeve being closed, each piston having a driving face at the trailing end thereof and the leading end of each piston being of reduced diameter to clear said opening in its associated sleeve, the length of the reduced diameter portion of each piston being such that in the extended position the leading ends of each adder piston extends beyond the leading end of its associated sleeve a distance which is proportional to the real value of a digital quantity represented by the particular adder piston assembly, the length of the reduced diameter portion of each adder piston being diierent than the length of the reduced diameter portion of every other adder piston, the leading end of each piston being liush with the leading end of its associated sleeve in the retracted position of that adder piston;

a load piston within said main bore and coaxial with said piston assemblies, said load piston being in abutment with the leading end of the nearest adder piston, the face of said load piston opposite that in contact with said nearest adder piston having an eiective piston area relative to the effective piston area of the driving face of each adder piston in the ratio of Psi-PR ZPS `iiuid bleed means communicating said second port means with said third port means;

-iirst check valve means for venting said first port means to said source of supply pressure whenever the pressure at said rst port means exceeds the supply pressure;

second check valve means for venting said second port means to said source of return pressure whenever the pressure at said second port means falls below said return pressure;

third port means in each adder piston in fluid communication with the driving face of the particular adder piston, said third port means being in fluid communication with said rst port means during the first half of each extension cycle and the second half of each retraction cycle, said third port means being in fluid communication with said second port means during the second half of each extension cycle and the first half of each retraction cycle;

a solenoid for each adder piston assembly;

a two-position valve for each adder piston assembly and controlled by the solenoid for the particular adder piston assembly, said valve in one position applying supply pressure directly to said irst port means, said valve in the other of its two positions supplying return pressure directly to said second port means, whereby to selectively induce acceleration and deceleration phases of said adder piston and said load piston for each extension and retraction cycle, substantially all of the energy supplied by said source of supply pressure during acceleration of said. adder piston and said load piston being returned to said source of supply pressure during deceleration of said adder piston and said load piston;

and electrical means connected to each solenoid for preventing initiation of any adder piston stroke in one direction before all of the adder pistons in the other direction have been completed.

References Cited by the Examiner UNITED STATES PATENTS 1,826,363 10/ 1931 Miedbrodt 60-51 2,699,757 1/ 1955 Tornkvist 92-51 X 2,969,042 1/ 1961 Litz 925 1 X 3,05 8,3 10 10/1962 Panissidi 60-51 3,141,388 7/1964 Brandstader 91-167 3,229,588 1/1966 Czernek 91-167 MARTIN P. SCHWADRON, Primary Examiner. P. T. CORBIN, B. L. ADAMS, Assistant Examiners. 

1. IN A DIGITAL ACTUATION SYSTEM, THE COMBINATION COMPRISING: AN ADDER PISTON; A LOAD PISTON COAXIAL WITH SAID ADDER PISTON AND IN FACETO-FACE CONTACT THEREWITH; AND MEANS FOR ACCELERATING AND DECELERATING SAID ADDER PISTON AND SAID LOAD PISTON IN EITHER DIRECTION ALONG THEIR COMMON AXIS INCLUDING MEANS FOR PRODUCING ON SAID LOAD AND ADDER PISTONS FORCES WHICH ARE EXACTLY EQUAL IN MAGNITUDE BUT OPPOSITE IN DIRECTION DURING 